Download FLAMES IN HORIZONTAL ELECTRIC FIELD

FLAMES IN HORIZONTAL ELECTRIC FIELD, DEVIATION AND
OSCILLATION
Rojin Anbarafshana, Hossein Azizinaghshb, Reza Montazeri Naminc
a
Rahe Roshd High School, I. R. Iran
Sharif University of Technology, School of Computer Engineering, I. R. Iran
c
Sharif University of Technology, School of Mechanical Engineering, I. R. Iran
b
Abstract
The current paper is an investigation on the behaviour and motion of flames in case
where the flame is placed between two charged parallel metal plates with different
charges. A physical experimental setup has been constructed to make precise
experiments. Observations give detailed information about the deviation of the flames
towards the negative plate, and in some special cases, the diffusion flames start
“oscillating”. Theoretical explanation has been proposed for the phenomena and has
been proved by the experiments in qualitative predictions. A numerical model has also
been developed based on the theory to be compared with the experimental data
quantitatively. This paper is based on the original solution of team of Iran on the 3rd
problem of IYPT 2011.
Introduction
Flames have been reported to be ionized gases and in some cases to be plasmas [1].
The fact of being plasma or not, depends on the flame temperature and the burning
material [1].
Asymmetrical division of the flame in two branches deviating toward different plates, in
case where the flame is placed between two charged parallel metal plates, has been
observed and investigated in few resources. A suggested explanation is the difference
in the mass of the positive and negative ions existing in the flame [2, 3]. In some
investigations it is mentioned that the free electrons in the flame existing because of the
ionization tend to react with the surrounding air molecules in the presence of the electric
field [3].
Other investigations have been made in different experimental situations, e.g. the effect
of vertical electric field on flame stability has been investigated numerically by Belhi et
al. [4] and it has been shown that flame stability increases in the presence of electric
fields due to the momentum source which exists because of the electric force exerted to
the ions within the flame (ionic wind).
We will discuss the effect of ionic wind on the shape and motion of the flame in
horizontal electric field, and will show that the difference of the mass of the positive and
negative ions is not a crucial matter in describing the phenomenon. The oscillatory
motion of the flame is also a subject of investigation, which has been initially observed
by the authors and no other references as we’ve seen, has reported such a motion.
Theory Base
In the situation where the flame is under the effect of electric field, Aside from
combustion process, some other chemical and electro-chemical reactions occur. In
order to determine the electric load within the flame; which is of significant importance,
these reactions must be well understood.
The first set of chemical reactions in the flame are those related to the main fuelOxygen reaction; a highly exothermic reaction. This reaction produces enough heat to
raise the temperature to obtain the ionization energy. The second reaction is the heat
thermal ionization, decomposing the molecules into positive ions and free electrons in
the high temperature zone. This reversible reaction causes free electrons to flow in the
boundaries of the ionized zone.
Combustion: Fuel + O 2 ⇒ CO2 + H 2O + Energy
Ionization: Fuel ⇔ Fuel + + e −
In the presence of the electric field the forces acting on positive ions and electrons are
in opposite directions, the charge separation will occur, and the contact of the electrons
with oxygen molecules increases. The electrons react with O2 producing O2-, and
separate from the flame [5].
−
Electron reaction: O2 + e − ⇔ O2
After this process the total flame charge will be positive in the presence of electric field.
Flame Deviation
Considering the positive and visible part of the flame as a system,
there would be an input stream of low-momentum particles
resulted from the combustion process entering from the lower part
of the flame, and an output stream of high-momentum cooled
particles exiting from the upper part of the flame.
There are three external forces applies to each particle in this
system; 1) Buoyancy Force, 2) Electric Force, 3) Air resisting
Force (in the case of instability). But since the system is not
isolated, there would be another force applied to the system due Figure 1: Applied Forces
to the difference in momentum of input and output particles. This
force is in the direction of the flame, toward the lower part of the flame (Figure 1).
In the case of steady deviation, these three
forces must be in equilibrium. According to
what we discussed; in the presence of the
electric field, as the bigger part of the flame
consists of positive ions, the major part will
be attracted towards the negative plate.
Meanwhile as a small part in the flame
consists of negative ions, a very small
portion will be attracted towards the
positive plate (Figure 2).
Figure 2: shape and behavior of the flame in the presence of
electric field
In case of plasma flames, the distribution of the electric charges causes a significant
internal electric field, cancelling the external electric field. This phenomenon is known as
the collective effect [1]. So in this case the plasma flame does not deviate at all.
Flame Oscillation
Because of being ionized, the flame is assumed to be conductive in high voltages [6]
(as our experiments confirm). Thus in the case when the flame can touch the plates
electrically, electrical discharge will occur and the flame will gain electrons from the
negative plate, losing the positive charge. This reduces the electric force, forcing the
flame to go back to its initial position. However, because of advection and the
continuous combustion reactions, it will be positively charged again. Therefore the
whole process will be repeated causing the flame an oscillatory motion.
Numerical Analysis
The numerical analysis of this phenomenon is based upon five assumptions; these
assumptions will lead us to develop a simple model that presents us some numerical
predictions of both deviation and oscillation of the flame. These assumptions are either
approved in reliable resources or can be easily observed to be true or they are
reasonable estimations.
We propose a simplified model for the flame based on these assumptions:
- The flame can be considered conductive in high voltages
- The flame is positively charged in electric field
- There exists an input and output electric current
- There is a force applied to the flame, in its inverse direction
- Electric and mass density are both uniform in the flame
There is an ingoing electric current, which has a constant amount (due to combustion
process), and an outgoing electric current, which is proportional to the electric load of
the flame. Considering these assumptions, the motion of the centre of mass of the flame
was simulated using a numerical method developed in MATLAB. The Euler method was
used to solve the ODE. Constants and coefficients were either measured or estimated.
Experiment Method
a) Experimental Setup
The experimental setup is consisted of several types of flame (candle or Bunsen
burner), two parallel aluminum metal plates connected to a high voltage device with the
maximum voltage of 14 KVs and adjustment resolution of 0.1 KV. Distance between the
two plates is precisely adjustable (1mm precision). In order to reach a uniform electric
field around the flame, size of the plates is large enough comparing to the size of the
flame (30 x 30 cm plates). The flame is placed in the middle of the two plates.
The corresponding behavior of the flame, under electric field was recorded using a highspeed camera (1000 FPS), placed exactly in front of the flame. Using this video, some
precise experimental data was extracted.
b) Video Processing
The recorded video has 1000 frames per
second, using MATLAB image processing
tool-kit, the relative position of the center of
area of the flame (estimated as the center of
mass) and the wick of the candle (lower part
of the flame) was found. The angle between
this vector and vertical line (Deviation
Angle) was measured in each frame (Figure Figure 3: MATLAB image processing. Left picture: Original picture,
Right picture:
Processed
picture experiments.
3). DA as a function of time and voltage was extracted
from
different
Experiment
Initially, some qualitative experiments were designed to evaluate the basic theory.
Streams of rising smoke were used in order to detect the flow
around the flame while it is oscillating. Flows of negative ions
toward the positive plate and also positive ions toward the
negative plate; were both observed (Figure 4); which verifies the
validity of ionic wind theory. These laminar flows also reject the
possibility of turbulent flows as the cause of oscillatory motion.
In case of oscillating candle flame, a layer of non-conductive
material was placed between the flame and the plates by coating
the aluminium plates, avoiding electrical discharge interaction Figure 4: Smoke Experiment
between the flame and plates. As a result, the oscillatory motion
of the flame was diminished, verifying the main reason of the oscillation as the electric
discharge.
Most of the experiments were done using a candle flame, however the behavior of other
flames was also observed. Pre-mixed flames mostly showed no oscillation at all. This is
a result of the sufficient ionization degree, which can keep a steady current within the
flame in the electrically touching condition with the plate. Some flames; e.g. the diffusion
petroleum flame, were highly turbulent; and unsteady motion could be observed even in
cases where no external field exists.
Three quantitative experiments were also
designed, in order to compare the
experimental results with the numerical model
predictions. Candle flame was used in all the
experiments.
a) Critical Voltage of Oscillation
According to previously proposed theoretical
explanation, the “Oscillation” occurs when the
flame
touches
the
plate
electrically
and Figure 5: Critical Voltage of Oscillation
discharges. For the flame to reach the plate there is a critical deviation angle and its
corresponding voltage. This voltage is a function of the distance between the plates.
The less the distance is, the lower voltage is required for oscillation to occur. The critical
voltage of oscillation was measured by gradually increasing the voltage in different
distances between the plates.
b) Deviation Angle Vs. Applied Voltage
Before the flame starts oscillating, it
deviates and reaches to a stable condition.
DA is proportional to the applied voltage.
By increasing the applied voltage DA will
also increase. A candle flame was placed
between the plates; distance between the
two plates was fixed, the applied voltage
was gradually increased and using imageprocessing method, the angle of deviation
was measured in each voltage.
Figure 6: Deviation Angle
c) Frequency of oscillation
it was observed that the oscillatory motion
of the flame has a well-defined frequency.
In a fixed distance between the plates, this
frequency was measured in different
voltages using image-processing methods.
The difference in high voltages can be
because of neglecting the air resistance,
which could get sufficient in high velocities.
According to the presented results of
experiments, the proposed theory is verified Figure 7: Frequency of Oscillation
in each step, qualitative experiments
confirm the basis of the theory and well-matched results of experimental data confirm
the numerical model to be an appropriate estimation.
References
[1] Gerald Rogoff, Paul Rivenberg; "Plasma and Flames - The Burning Question", Coalition for Plasma
Science 2008
[2] Hoo Sze Yen, "Physics SPM 2008", Chapter 7:Electricity, Page 3
[3] Yew Kok Leh, William Chia Wong Fei, "Visual Pelangi Kbsm 2010 Physics Spm", Page 35
[4] M. Belhi, P. Domingo, P. Vervisch, "Effect of Electric Field on Flame Stability", UMR-CNRS-6614CORIA, Campus du Madrillet, Avenue de l'Universite,BP8, 76801 St. Etienne du Rouvray Cedex,
FRANCE
[5] J.M. Goodings, D.K. Bohme, Chun-Wai Ng, " Detailed ion chemistry in methane-oxygen flames. II.
Negative ions", Department of Chemistry, York University, 4700 Keele Street, Downsview, Ontario, M3J
1P3, Canada